The continued miniaturization of semiconductor devices is being blocked by limits imposed by the physics of semiconductor operation and the inability to laterally define yet smaller devices. Much interest has arisen in quantum well effects in which the dimensions are of the order of less than 50 nm for the GaAs/GaAlAs and InP/InGaAs groups of semiconductors. At this size, energy gaps can be controlled by the thickness of the regions. Quantum well layers can be fairly easily fabricated by epitaxially growing a thin semiconductor region on a semiconductor of the same crystal structure but of a higher bandgap. Quantum wires however present more promise but more difficulty. A quantum wire is a long semiconductor region with a cross-section in two dimensions of the order of 50 nm or less. A quantum wire can be used as an electrical conductor, as an optical fiber (assuming proper cladding layers) for light of energy less than the bandgap of the quantum wire or as a small laser (assuming proper charge injection) in which the lasing wavelength depends on the thickness of the quantum wire. However, standard lithographic processes have much difficulty in laterally defining such small features.
One promising technique has been developed by Elyahou Kapon of Bell Communications Research and relies on differential growth rates of different crystal faces of a binary or higher order semiconductor, such as the III-V compounds GaAs and GaAlAs having a zincblende crystal structure. His technique is described in a number of technical articles, such as "Patterned quantum well heterostructures grown by OMCVD on non-planar substrates: applications to extremely narrow SQW lasers" by Bhat et al appearing in Journal of Crystal Growth, volume 93, 1988 at pages 850-856 and "Lateral patterning of semiconductor superlattice heterostructures by epitaxial growth on nonplanar substrates" by Kapon et al appearing in Growth of Compound Semiconductor Structures, Proceedings of SPIE, volume 944, 1988 at pages 80-91. In general, Kapon lithographically etches a groove in a predetermined direction on a predetermined major crystal face of a GaAs crystal. The lithography limits the groove width to the order of no less than 0.3 .mu.m even for advanced techniques, far above the dimensions for quantum effects. Subsequent layers of other III-V compounds of different electrical characteristics are epitaxially deposited in the groove and surrounding areas by techniques such as molecular beam epitaxy (MBE) or organometallic chemical vapor deposition (OMCVD). It has been observed that the growth rate of the material depends upon the crystal face upon which the material is deposited. By careful selection of the crystal faces in the groove, dependent upon the growth technique, the grown structure in and adjacent the groove can be made to have distinct lateral variations. In the above technical paper by Bhat et al, they have shown that crescent-shaped quantum wires can be grown.
The Kapon technique suffers from the disadvantage of producing quantum wires of highly irregular shape, specifically crescent-shaped. Since the bandgap depends upon the thickness, the bandgap varies over the irregular cross-section of the wire. The device characteristics are dependent on the exact dimensions of the crescent, which in turn are sensitive to growth parameters. Furthermore, the technique is limiting in that the quantum wire is formed near the bottom of the groove, thus reducing the flexibility in attaching further structure to the quantum wire.
Another technique that can grow quantum wires is the growth of epitaxial layers on vicinally cut GaAs crystals, as developed by Petroff. This technique is described in a technical article by Petroff et al entitled "Structure of AlAs-GaAs interfaces grown on (100) vicinal surfaces by molecular beam epitaxy" appearing in Applied Physics Letters, volume 45, 1984 at pages 620-622 and in another technical article by Gaines et al entitled "Molecular-beam epitaxy growth of tilted GaAs/AlAs superlattices by deposition of fractional monolayers on vicinal (001) substrates" appearing in Journal of Vacuum Science and Technology B, volume 6, 1988 at pages 1378-1381.
As illustrated in FIG. 1, a crystalline ingot of GaAs is cut into a wafer at a vicinal angle .alpha. with respect to the [001] direction and toward the [110] direction and the wafer is polished and cleaned. The vicinal angle is relatively small, for instance 2.degree.. The result is a series of steps or microsteps with step sides 10 formed in a GaAs substrate 12. The step height d (0.283 nm for GaAs and 0.293 nm for GaAs and 0.293 nm for InP) is directly related to the interatomic bonding lengths, that is, it corresponds to an atomic monolayer. For known zincblende semiconductors, d=a.sub.o /2, where a.sub.o is the cubic lattice constant. That is, in these materials a microstep is formed across a bilayer of a layer of Ga and a layer of As. Bottoms 14 of the steps are oriented along the [001] direction.
In a first crystal growth deposition, a one-half monolayer 16 of AlAs is deposited by MBE. It is important for the Petroff technique that monolayer growth proceeds from corners 18 of the steps. Control of the fractional monolayer deposition is by timing the exposure to the MBE beams. In a second crystal growth deposition, a one-half monolayer 20 of GaAs is deposited by MBE. Thereafter, the AlAs and GaAs depositions are repeated N times. There results GaAs quantum wires of a height N.multidot.d and a width t=d/(2.multidot.tan.alpha.). The width can be further controlled by varying the fractionality of the GaAs monolayer deposition.
The Petroff technique is thus capable of growing structures of just a few atomic lengths. However, the small scale is one of its disadvantages. If the minimal vicinal angle .alpha. is 0.1.degree., then the maximum width for one-half monolayer of the structure shown in FIG. 1 is 81 nm and the period of the structure is 162 nm. The width is acceptable but the period has an upper limit determined by the minimum vicinal angle that may be too small for integrating with lithographically defined features. The primary control over thickness and period is the vicinal angle .alpha.. This angle is determined mechanically and not lithographically. The fractional monolayer depositions are difficult to control to exactly one-half monolayer. As the sum of the two fractional depositions departs from one monolayer, the structure of FIG. 1 becomes tilted, assuming that the fractions remain constant. For large number N of depositions, the fractions change and the structure weaves and some of the wires can disappear. The Petroff technique assumes a perfect regularity of the microsteps in the starting surface, which is hard to obtain in practice.